During the late 20th century, Medellín, Colombia, was widely considered the world’s deadliest city.

The second-largest city in Colombia, Medellín was gripped by violence fueled by corruption and drug cartels, like the one run by Pablo Escobar. In 1991, there were 6,349 murders in the city of 2.5 million. Today, that rate has been cut by about 80 percent, according to global and national statistics. As its people like to say, Medellín has moved “del miedo a la esperanza” from fear to hope.

And it has gone from a city with pockets of immense wealth surrounded by desperate slums to an economically and culturally integrated metropolis, a global destination for tourists.

The person many see as most responsible for the dramatic turnaround in Medellín is Sergio Fajardo, the city’s mayor from 2004 to 2007, and the governor of Antioquia Department, the country’s largest province, from 2012 to 2015. The son of a wealthy and prominent Medellín family in the city, Fajardo was a mathematics professor and he took a mathematician’s approach to identifying and addressing the three most significant problems plaguing the city: violence, inequality and corruption.

He will speak at 10:45 a.m. Tuesday in the Amphitheater on the topic “The Power of Ingenuity,” as part of this week’s theme “The Future of Cities.” Carolina Barco, the former Colombian ambassador to the United States and a former foreign minister of her country will join him as an interviewer and speaker.

Fajardo ran for mayor in 2000 as a completely independent candidate, aligned neither with the left or right, although he held some decidedly progressive convictions, especially about inequality. A newcomer to politics at 43, Fajardo inspired a movement called Compromiso Ciudadano, and brought together experts from the fields of academia, nongovernmental organizations, business, law and culture to guide it

“We had a simple strategy,” Fajardo said. “We said we have to walk the city and meet with people in all of the neighborhoods. We put the city in our skin, in our hearts, in our brains and in our senses. Our group was always outside of politics, outside of the traditional parties. We claimed, and still claim, that politics itself has to be transformed. We didn’t win, but we had an impressive showing.”

The movement stayed together and grew stronger over the next four years, with its leaders operating as a sort of shadow cabinet. Stressing “the four basic values of dignity, capability, respect and recognition,” the group meticulously planned solutions to Medellín’s considerable problems. When Fajardo was elected in a landslide in 2004, he hit the ground running.

Violence in the city was already declining in 2004 because of a truce reached between the national government and the right-wing paramilitary groups affiliated with drug cartels that had a stranglehold on the city. Through various amnesty programs, the paramilitaries were demobilized and the killing largely subsided. To keep the violence from coming back, Fajardo focused on creating employment and entrepreneurial opportunities, which allowed people to re-enter civil society.

The key to fighting inequality and corruption, Fajardo believed, was education. Not just new classrooms and books, but a whole new approach to how education was delivered that could be owned by the people it served.

To this end, Fajardo’s administration built a network of complexes consisting of libraries, schools and parks in the poorest neighborhoods in the city. Heralded with the slogan “the most beautiful for the most humble,” the facilities are dominated by striking, modernistic buildings that house classrooms, libraries and other public buildings surrounded by recreational space and tied together with the rest of the city through improved and more extensive public transportation using trolley cars, tramways and trains.

These projects were paid for by largely eliminating the systemic corruption that had become ingrained in both the conservative and liberal political parties that had traditionally held power.

“We said we are not going to build coalitions in the traditional way,” he said. “We invite you to work with us, but not in exchange for a position or a contract. We are going to build political capital through trust.”

It helped to have had an electoral mandate in winning over city council approval, but Fajardo also made sure to share the credit for his efforts with local officials, whose popularity among the people benefitting from the programs rose with their success. The idea that everyone had a stake in these changes took hold.

Money was also saved by holding public competitions among local and regional architects to design and build the new facilities, which have won international awards for their beauty and utility.

People who lived in the slums where the libraries and parks were built were proud of the shining new additions to their neighborhoods and took advantage of them. There were virtually no instances of vandalism. School attendance soared. And people from wealthier parts of the city began to venture into areas they had previously been afraid to go.

The experiment has become a model for cities throughout Latin America and the world. In 2013, The Wall Street Journal named Medellín “Innovative City of the Year.”

“We changed the skin of the community, socially, culturally and physically,” Fajardo said. “Architecture is a powerful tool and can create powerful symbols. It makes hope tangible. We created a different city with a different attitude. It is not a paradise, there are still many problems, but things are improving.”

Colombian law limited Fajardo to just one term as mayor, but his legacy has been continued by former staffers. And he himself was able to build on his successes after he was elected governor.

Colombian coffee is world famous, of course, and Antioquia is the heart of the nation’s coffee-growing country. Working with the growers, Fajardo’s administration has brought innovative methods to refining and growing specific kinds of coffee beans, which has resulted in higher profits for growers and higher wages for workers.

It has also spawned a boom in high-end cafés in Medellín, drawing even more tourists to a city that was written off 25 years ago but now has been resurrected as both a vacation hot spot and a good place to live.

Fajardo plans to run for president in 2017, and is now tied at the top of the polls. After decades of civil war, the government is on the verge of signing a historic peace agreement with the Revolutionary Armed Forces of Colombia, or FARC.

“We are turning the page and ready to write a new page in our history,” Fajardo said.

He still sees himself more as a professor than a politician, but he is proud of the accomplishments achieved over the past 17 years.

“I never thought we would come this far so quickly,” Fajardo said. “People can do extraordinary things, starting at the simplest level. Nothing is impossible. We have gone from fear to hope, but hope has to be fed; it has to be nurtured. Things can change in a second.”

David Geary has been a reporter and editor for more than 40 years. He retired in 2015 from The New York Times, where he was the late-night news editor. He also has worked for The Boston Globe and other newspapers. He currently freelances for the Times and for The Trace, a nonprofit journalism website focused on gun violence and gun policy in America. He lives in Westfield with his wife, Karin Henry.